Figure REF  shows the distance between the ground truth and the original reference image and the distances of LK and DFE methods for tracking the nose tip under bike condition temporally.
We believe that the large error from LK is due to the ground truth having a very large displacement from the original feature.
This displacement causes the new feature to be deformed and thus making the optical flow equations hard to solve.
As mentioned previously, an assumption from the LK method is that the motion is small [1]}.
The error in LK has a correlation coefficient of 0.52 with the distance of the ground truth to the original reference feature.
Determining whether a correlation is strong is subjective but a correlation coefficient above 0.5 is often considered moderate to strong [2]}.
Thus we believe this to in part explain the large errors.
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